I Cystine

Preparation by acidic proteine hydrolysis (e. g. of keratines) with following fractionated crystallization (obtained in commonly with i.-cystine). 

Fui. 3. Absorption spectra of cystine and cystine peptides (Otey and Greenstein, 1954). Absorption curves in 0.01 N HCI of (I) l.-oystinyl-n-cystine and o-cystinyl-i.-cystine (II) L-cystinyldiglycine (III) cyolo-L-cystinyl and L-cystinyl-L-oystine (IV) n-cystine (V) glycylglycine and (VI) glycine. 

D-aspartic acid, L-/i-methyIaspartic acid D-cysteine, iV,iV -dimethyl-i,-cystine D-glutamic acid... 

In some circumstances it is possible to express protein requirements in terms of total protein alone, for example if animals are fed on a limited range of foods of known amino acid composition. This is the case for growing pigs in the USA that are fed mainly on maize and soya bean meal. Such a simplified approach cannot be sustained when a wider range of foods and by-products are used, and when diets must be formulated not only to maximise growth but also to optimise carcass composition. Feed compoimders therefore formulate pig diets to meet the requirements for at least three amino acids (lysine, methionine -I- cystine and threonine).They will also take into accoimt the availability of certain amino acids, assessed from digestibility at the terminal Ueiun (as explained in Chapter 10). 

Conversely, according to observations of du Vigneaud, Loring, and Craft (133) with rabbits, the sulfur of acetyl and, formyl derivatives of l-cystine is oxidized just about as in the case of i-cystine itself. However, the sulfur of the corresponding derivatives of d-cystine is considerably more resistant to oxidation than the sulfur of d-cystine. 

Value reported for V,Ar -diglycyl-i.-cystine (Steinrauf et al., 1958 Yakel and Hughes, 1954). 

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